The present invention relates to the formation of isolation trenches between semiconductor devices, and more particularly to local oxidation of silicon (LOCOS) nitride capping of a polysilicon-filled isolation trench to achieve effective device isolation.
Fabrication of semiconductor devices often requires electrical isolation between adjacent devices. Particularly in high voltage applications, one way to provide such isolation is to form deep, polysilicon-filled trenches in the substrate between devices. This is achieved by etching a deep trench in the substrate, depositing oxide in the trench to line the trench wall, filling the remainder of the trench with polysilicon, and applying field oxide over the filled trench. However, the application of field oxide over the filled trench can result in a wedge of oxide penetrating seams in the polysilicon filling the trench, creating stress that degrades the performance of the trench isolated devices.
Some efforts have been made to protect the polysilicon filled isolation trench with a LOCOS nitride cap that covers the entire region of the isolation trench. While this provides protection against oxide penetrating seams of the polysilicon, it also causes the device footprint to expand by displacing the field oxide further outside of the isolation trench region, and/or can result in reduction of the electrical breakdown threshold of the isolation trench due to the formation of small gaps between the polysilicon trench fill and polysilicon stringers that form over the isolation trench.
It would be useful in the art to form polysilicon-filled isolation trenches in such a way as to provide good mechanical and electrical performance.